Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the coordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, resonance frequency, or MR frequency). From a macroscopic point of view, the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field of the RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precession about the z-axis. This motion of the magnetization describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°). The RF pulse is radiated toward the body of the patient via a RF coil arrangement of the MR device. The RF coil arrangement typically surrounds the examination volume in which the body of the patient is placed.
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of the aforementioned RF coil arrangement of the MR device. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the RF coil arrangement then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation.
The band pass birdcage resonator (also referred to as birdcage coil) is a well-known concept for the RF coil arrangement in MR imaging. Such a birdcage resonator comprises a plurality of rungs arranged in parallel to a longitudinal axis of the examination volume surrounded by the birdcage resonator. Each rung comprises a rung capacitance Crung. Two end rings are arranged at the opposite ends of the rungs, wherein each end ring comprises a plurality of ring capacitances Cring. Each ring capacitance Cring interconnects a pair of adjacent rungs. Such a conventional design of a birdcage resonator as commonly used in MR imaging is shown in FIG. 1.
The resonant frequency of the multiple resonant modes of the band pass birdcage coil are determined by the various geometric properties of the coil (which influence its inductance properties) and the values of capacitances, Cring and Crung, that are placed in the end ring segments and the rungs respectively. For any given geometry, while keeping the ratio of Cring/Crung fixed, it is possible to tune the coil structure to place a desired resonant mode at the required Larmor frequency. This is done by manipulating the values of Cring and Crung in a coupled manner. It is observed that the ratio of Cring/Crung determines the relative frequency separation of modes while the absolute values of Cring and Crung determine the absolute frequency. By selecting a ratio of Cring/Crung it is possible to manipulate the multiple resonant modes of the coil as required. The ISMRM 1997 p. 176 abstract ‘The bandpass birdcage resonantor modified as a coilray for simultaneous MR acquisition’ mentions that the ratio of rung-capacitors to ring-capacitors is a critical parameter to determine the low-pass or high-pass characteristics of a bandpass birdcage coil. In the birdcage coil known from ISMRM 1997 p. 176 abstract adjacent meshes are isolated.
While in FIG. 1 the capacitances are shown as discrete capacitors, the values Cring and Crung may be realized using multiple capacitors in series or parallel.
In a typical MR imaging system, a band pass birdcage resonator is employed, wherein the design of the birdcage resonator uses capacitance values Cring and Crung which realize the common so-called two-port birdcage tuning in which only a single uniform resonant mode is tuned to the Larmor frequency. In this case, the coil structure can be driven in quadrature by supplying RF power via two RF drive ports located physically 90° apart. In the case of quadrature excitation, the magnitude of the RF current is equal in each rung, while the phase of the RF current increments from rung to rung in equal increments from 0° to 360°.
It is generally desirable to have a relatively uniform homogeneity of the generated RF field (B1 field) for excitation of magnetic resonance throughout a cross section of the imaged portion of the patient's body. However, as the MR frequency increases with increasing strength of the main magnetic (B0 field), this becomes more difficult due to conductive losses and wavelength effects within the body of the patient. On examining the performance of a conventional birdcage resonator with respect to excitation uniformity it is observed that at high frequencies (>128 MHz) dielectric based standing wave mechanisms severely affect the B1 field uniformity.
Multi-channel transmit MR imaging has been accepted as a standard method of operating volume RF coils to achieve a relatively uniform B1 field. In the basic example of multi-transmit, the volume RF coil arrangement is split into many independent resonator elements. The RF signal for generating the B1 field is then supplied to the RF coil arrangement via RF drive ports being connected to the individual resonator elements. The RF power and the RF phase applied to the different RF drive ports can be controlled individually in order to optimize the uniformity of the RF field (so-called RF shimming).
One way of realizing a multi-transmit RF coil arrangement is known from U.S. Pat. No. 6,043,658. In this approach the Cring and Crung capacitance values of a birdcage resonator are selected such that all resonant modes are tuned to a single frequency. The effect of this is that the individual meshes of the birdcage resonator behave as electromagnetically decoupled individual elements. Each mesh can then be treated as an independent resonator element, which is supplied with an individually controllable RF signal in accordance with the multi-transmit scheme to achieve a uniform B1.
However, the ability to realize the degenerate tuning of a birdcage resonator (as known from U.S. Pat. No. 6,043,658) depends on the dimensions of the coil and the number of rungs. It has been observed that in the case of a large bore design with a closely fitting RF screen it is difficult, even impossible, to achieve the conventional degenerate tuning in practice.